1. Field of the Invention
The present invention relates generally to wireless communication systems and, more particularly, to a system and method for receiving and processing forward link AMPS signals.
2. Description of the Related Art
Wireless communication systems have increased in number and complexity in recent years. It is common that a plurality of wireless service providers may be operating in the same geographic region with overlapping areas of coverage. Furthermore, different wireless service providers may be using different technologies for their systems. Some service providers may use code division multiple access (CDMA) wireless systems while others may use such systems as time division multiple access (TDMA) wireless systems. However, certain regions may only have an analog system available such as the Advanced Mobile Phone Service (AMPS) system. Thus, a wireless communication device (WCD) designed for the CDMA system may not be able to communicate in a region providing only AMPS service.
To solve such problems, many advanced WCDs now have a dual mode capability that allows the WCDs to communicate in both the CDMA system and the AMPS system. The AMPS system is a first generation analog cellular system that uses frequency modulation (FM) for radio transmission.
A typical dual-mode receiver architecture is heterodyne, which uses an analog intermediate frequency (IF) automatic gain control (AGC) circuit consisting of a variable gain amplifier (VGA) in the IF strip to process received forward link signals (i.e., a signal from a base station (BS) to a WCD). In a typical AMPS receiver, a radio frequency (RF) signal detected by the RF stage in the receiver is mixed or translated down to the intermediate frequency. The IF stage may perform additional amplification and/or filtering. However, a new trend in wireless communication devices, particularly in dual-mode CDMA wireless communication devices, is to mix the output of the RF stage directly to baseband frequencies in a zero IF (ZIF) architecture or a low-IF architecture. The ZIF architecture saves cost and circuit space by reducing the total part count. Specifically, in the ZIF architecture, the IF section is removed including filters, VCO (voltage controlled oscillator) etc. However, the utilization of the ZIF architecture in a traditional CDMA receiver when combined with an analog AGC as used in a typical AMPS receiver will be difficult because of DC offset problems and mismatches associated with analog circuits. Thus, a CDMA receiver with a ZIF architecture must use a digital AGC to overcome such problems.
However, traditional AMPS receivers use analog AGCs to process the received forward link signals, but having both analog and digital AGCs in a dual-mode WCD increases the number of parts, manufacturing cost and circuit space.
Therefore, there is a need for an AMPS receiver that is able to use a digital AGC to process the received forward link signals and share the digital AGC with the CDMA receiver to reduce the number of parts, manufacturing cost and circuit space. The present invention provides this and other advantages as will be apparent from the following detailed description and accompanying figures.